NVIS Explained, I

Administrative note: The following was sent by Keypounder, the nom de plume that should be familiar to a few readers. A longtime radio amateur and communications specialist in his own right, he’s generously submitted this article to me, and in my opinion, is among the best I’ve read on the topic for public consumption. Due to length and volume the article is posted in three parts. Contained is a keen rundown of how Skywave propagation works, the requirements for NVIS on the HF bands, the history, antenna modelling with EZNEC, and fantastic examples of antennas in the field for this task. It is meant to be a much more in depth article than my previous work on the subject, which was intended for beginners.

NVIS is a highly misunderstood term parroted by many ‘preppers’ and militia-types with usually very little demonstration or explanation of said skill. It is for this reason the previous was written, and for this reason Keypounder has given his time to explain it further and give a very practical explanation of skywave propagation. I will re-iterate that these skills, along with Land Navigation, are among the most perishable and most difficult to learn- under duress, is near impossible. So for those of you who feel you’ll do it when ‘the time comes’, you’ll be sadly mistaken.

Please folks, try this at home.

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NVIS 201, or peeling the onion on advanced HF techniques

About the author:

“Keypounder” is the pen name of an amateur radio operator first licensed in the 1970s. He is a long-time student of radio propagation, and antenna design and construction, having written an article on low band listening antennas for Signal-3. His interests also include, in no particular order, emergency communications; rag-chews; HF contesting on both CW and SSB; and direction-finding techniques.

Caveat:

This article is not intended for the beginner; it assumes that the reader has basic knowledge of radio electronics and is a licensed amateur operator with an FCC General Class license, or the foreign equivalent. It is NOT possible to gain skill in NVIS operation, the subject of this article, without actually operating. I could spend several pages detailing all the reasons I think unlicensed operation is a bad idea, but if you are thinking about operating without a license, please don’t. A license these days is easy to get; you don’t even have to learn Morse code.

Technician Class licensees who do not operate CW, don’t have the frequency privileges to operate NVIS, but there are a lot of new General Class operators and even some old-time Advanced and Extra Licensees who can benefit from this information. Finally, this material is presented with the thought that NVIS will be most useful in a grid-down emergency situation, where the current VHF and UHF repeater systems are not available.

With that out of the way, let’s get on to the meat of the matter!

I have studied NVIS quite a bit over the years, and I have had a fair amount of experience with it, too. It wasn’t always called that; the current name really came into vogue in the mid 1990’s, but the idea has been around a long time, as we’ll see. Many people have a variety of different, often conflicting, ideas about what and why NVIS is. I was told a long time ago by a very wise and very smart person (and no, they are not the same thing!) that the truth is a lot like an onion; every time that you peel away a layer to get to a deeper understanding of the truth, there is another layer underneath that. Well, folks, I can tell you that I’m not going to get all the layers peeled, but maybe we can shave a layer or two off and get at a better understanding of what NVIS is, why NVIS is, and how NVIS can be of benefit.

NVIS stands for Near Vertical Incidence Skywave, and what that means is that the radio operator bounces a radio signal off the ionosphere more or less directly overhead, and is able to send and receive signals to other stations out to perhaps 300 to 500 miles away, depending on the time of day and the state of the ionosphere.

Near Vertical Incidence Skywave is a specific operational mode for HF radio communication and the distinguishing characteristics are:

Intentional limitation of communication range;

Use of high angle lower HF radio emissions reflected off the F layer(s) of the ionosphere; NVIS is a type of skywave communication.

Generally, use of low power and reduced height antennas.

Use of frequencies from 1.8 to perhaps as high as 10 mHz in North America (possibly higher during solar maxima or close to the equator); frequency used depends on solar flux, time of day and other factors.

Let me digress for a moment to clarify some things about HF radio communications. There are three main types of radio propagation:

Ground wave, where the signal is usually vertically polarized (perpendicular to the surface of the Earth) and usually low frequency. Ground wave is the propagation mode for most daylight AM radio broadcast stations; the radio waves actually hug the surface of the Earth and travel along the ground. Geologic discontinuities like mountains, rivers, and deep gorges attenuate ground wave propagation, as does the absorption of the RF by the ground. This propagation is inversely proportional to frequency; the higher the frequency, the quicker it attenuates.

Sky wave, where the signal travels from the transmitting antenna to the receiving antenna by reflecting one or more times off any one of several layers of the ionosphere. It may include reflection from the ground in between as well. This includes NVIS as well as skip communication, auroral reflection, Sporadic Eskip, and tropospheric ducting, among others. This type of propagation is the most common propagation mode for HF radio communication. While the signals may be horizontally, vertically or circularly polarized by the transmitting antenna, the reflected signal can arrive at the receiving antenna with any polarization due to their reflections from the ionosphere and/or the earth. To reiterate, NVIS is a skywave propagation type, because the signal bounces off the ionosphere and comes back to Earth.

Line of sight, (LOS) where the transmitting antenna and the receiving antenna are in view of one another, and propagation is not by means of ionospheric reflections or by ground wave. This propagation can be horizontally, vertically or circularly polarized. LOS signals can be bounced off structures or geologic features like cliffs, too, or refracted off mountain tops, and while the purist might argue that these are not line of sight anymore, the generally accepted definition includes such reflections. Much VHF and UHF communication is LOS, although there are exceptions.

NVIS does not use VHF or UHF frequencies, does not use multiple hops or cover great distances, does not require high antennas, and does not typically use high power.

History of NVIS-

NVIS was first discovered or developed by the German Army in World War Two, while they were engaged with the Soviet Union on the Eastern Front. What they found was that while their excellent upper HF/lower VHF radios, developed during the late 1930s during the run-up to the solar peaks of Cycle 17 (1937 to 1939) worked very well for long haul HF communication, they had great difficulty using these radios for intermediate communication between groups or columns; their columns were often too far away for LOS communication and too close for HF skip. Amateur operators (Hams) today call that being in the “skip Zone.”

In modern technical terms, the F layer direct reflection frequency (FoF2) required for short range skip would have been too low for many of their radios. So the Germans started using a lower frequency radio originally intended for infantry use and created special horizontal dipole and loop antennas mounted on their vehicles to give their lower frequency signals maximum amplitude directly up. Here is a photograph of Heinz Guderian’s command vehicle showing a NVIS cage antenna. One can also see the extendable mast used for VHF in the center.

During D Day and again during the Vietnam War, the US military also had issues keeping in touch with various elements of their forces, especially in rugged highlands terrain. The answer was, again, NVIS, using low frequency low elevation horizontal antennas to maintain communications. Today, NVIS communication is practiced and used by military organizations and their affiliates, as well as various members of the preparedness community, to provide reliable, fast, and secure communications with lower probability of DF location.

Man that is pretty interesting information. Reminds me of something I’ve long wondered about.
For a time when I was a kid we lived on a farm up located right up against a nuclear tipped Nike sites. We would sneak in under the fence to hunt and fish, it was a pretty big reservation with lots of square miles of bomb proof concrete missile bunkers tucked in the sides of two ridges, target ranges, army vehicles and such. They had a belt fed gunnery range, we loved to creep around and watch the solders practice, we never got caught, probably a good thing being they had nukes and all. It was great hunting because nobody was hunting it, and we would sit up on the ridge and pop every coyote and feral dog we could see. But one thing always puzzled me we would pass getting to a nice trout stream. It was a perfectly groomed flat field with a beautiful thick lawn in the middle of a white pine thicket, 2 football fields wide by two long. At one end was a huge horizontal duct like sheet metal “horn”, fabricated out of square sections, it tapered down to about 4 ft square from the mouth that was around 20ft square. The field was studded with telephone poles about 20 ft tall, and strung between the poles was coax cable with baluns in the center of each run, each run was about 600 or 300 yards alternating, the height above ground was about 10 feet or so, insulators with strain cable loops at the poles, the center cables all ran back through ducts to the building the “horn” sat up on. The horn had a gimbal to turn and tilt it. Not a lot, looked like about 20 degrees in each axis. If I remember correctly the horn faced west. A family friend adult after us kids described this antenna farm said it might have been for talking on low frequency radio with submarines.
Regardless it sure looked a lot like the NVIS antennas described here.
Any ideas on what this was?

That makes sense if it was part of the DEWY line and the early warning system. They had microwave towers everywhere up in NewEngland when we was kids. There was all the military industrial manufacturing like Raytheon, Honeywell, Kodak, Poliroid, MIT, Drapper Labs, most produced high-tech and nuke parts or associated hardware, there was Otis, Hanscom, Pease and Drum, Pease was a SAC base, Hanscom flew fast movers, Portsmouth Navel Yard, all that stuff. We lived up in the northern NH mtns, there was large radio antenna towers everywhere, long gone now.

I do not know for sure, but what you saw sounds as if it may have been a relative of a “fishbone” antenna, one sort of wave antenna commonly used for listening on the MF and HF bands. I’ve never seen a waveguide horn for HF, but it would have to be large!

Near Remington Virginia there is a huge federal antenna farm that goes on for over a mile; you can seen all sorts of antennas there, too.

Wave guide? It was large. We climbed into it once, there was an access ladder bolted to the concrete block house it was mounted up on. We saw guys sweeping it out couple times, it had bird dropping in it when we checked it out, I’d say it had to be 20 high by 30 foot wide at the opening. That close to the size required for HF wave form? Or low frequency like scout mentioned?

It was aimed straight over the NVIS farm, in the long direction.
There was a microwave tower not too far set off at an angle, about half a mile. Had those big flat plate antennas only on one side, pointed relative north.

well, a 20 meter wave guide would be about 66 feet wide by 33 feet tall; a 10 meter guide would be 33 x 16, if I remember my microwave designs right (rectangular waveguides are usually 1 wavelength by 1/2 wavelength, IIRC.) Microwave experts are invited to correct me if I am wrong. If that was the feed to the array you described, then it was not an NVIS antenna.

The old memory is fuzzy, but 33×16 is pretty close. Would they have used microwave for comms for a Nike missile base that had nuclear warheads back in the day? I had heard years later there was only a few nuke tipped Nike sites around the US. The rest where conventional explosive warhead tipped. (The family jokes about the utility of duck and cover, bending over and kissing your hind end goodbye… When the Cuban missile crisis was on it was pretty much expected NH would be a glowing crater in the earth being we where surrounded by all the air bases and cold war manufacturing.)
There was a conduit “pipeline” ran back to an underground complex from the antenna farm. We never creeped to close to that. It looked real serious to us kids. A couple football field sized concrete slab, with nothing on it but an all glass telephone booth looking structure with nothing in it but a stairwell leading down, a ramp that they would drive ten wheel army trucks down that had a lid they would raise and lower to cover the ramp. There was a series of concrete quonset hut shaped bunkers set underground with a rail system leading from the bunker complex to a number of other concrete slabs where the missile launchers would be hidden till needed. We where afraid to go over near those also. It was called the Hanson Reservation, in Maynard Massachusetts. Our farm in NH burned down, my grandad leased this farm next to the Nike site for a couple years till we got the old place rebuilt. So there I’m sure lots of things I’m missing about the place because we didn’t live there all that long and I was pretty young.

Only noticed this today and quickly skimmed it, you might be able to a certain some info about what we are discussing. Strikes me as quite a lot of history involved in the Nike system. I didn’t notice anything in particular regarding comms and antennas. Maybe the antenna farm we seen was for something else entirely and just occupied part of the missile site? Still real interesting piece. Brings back lots of memories of a time that seems like a thousand years ago.